Surface-active compounds, or surfactants, are widely used as wetting agents and detergents in industry, pharmaceutics, and in the home. Surfactants are compounds which, while soluble in a given liquid, tend to accumulate in the interfaces between this liquid and air, another immiscible liquid, or a solid. Because of their high concentration in these interfaces, surfactants lower the corresponding interfacial tensions or free energies.
Surfactants are amphiphiles because they contain a hydrocarbon (hydrophobic and lipophilic) portion and one or more ionic or otherwise strongly hydrophilic groups in the same molecule. This dual nature causes them to be preferentially adsorbed at air-water, oil-water, and solid-water interfaces, forming oriented monolayers in which the hydrophilic groups are in the aqueous phase and the hydrocarbon chains are pointed towards the air, in contact with the solid surface, or immersed in the oil phase.
As increasing amounts of a solid surfactant are dissolved in a beaker full of water and its concentration in solution increases, its monolayers adsorbed at the air-water and glass-water interfaces become more and more crowded until they are so tightly packed that further occupancy requires excessive compression of the surfactant molecules already in the two monolayers. Further increments in the amount of dissolved surfactant beyond that concentration cause amounts equivalent to the new molecules to aggregate into micelles. This process begins at a characteristic concentration called the critical micelle concentration (CMC). The CMC value is of practical importance since it is the minimal concentration of surfactant required to solubilize hydrophobic molecules in water.
From the time the CMC is reached, the concentration of monomeric or nonassociated surfactant molecules hardly increases, rising only slightly above the CMC, but the concentration of micellar or associated molecules increases in direct proportion to the increase in overall surfactant concentration. In dilute concentrations, the micelles are approximately of the same size. Increments in surfactant merely increase the number of micelles.
Surfactants are generally classified according to chemical structure. For convenience, they are categorized according to their polar portions since the nonpolar portion is usually made up of alkyl or aryl groups. The major polar groups found in most surfactants may be divided as follows: 1) anionic; 2) cationic; 3) amphoteric; and 4) nonionic.
The most commonly used anionic surfactants are those containing carboxylate, sulfonate, and sulfate ions. Those containing carboxylate ions are known as soaps and are generally prepared by the saponification of natural fatty acid glycerides in alkaline solution. The most common cations associated with soaps are sodium, potassium, ammonium, and triethanolamine, while the chain length of the fatty acids ranges from 12 to 18.
Amphoteric surfactants include those containing carboxylate or phosphate groups as the anion and amino or quaternary ammonium groups as the cation. The former group is represented by various polypeptides, proteins, and the alkyl betaines, while the latter group consists of natural phospholipids such as the lecithins and cephalins.
Most of the surfactants produced by the chemical industry are based on petrochemicals. A number of efforts to use carbohydrates as bulk raw materials for synthesis of non-ionic surfactants have been reported. The amphiphilic behavior of petrochemicals is caused by the presence of the hydrophilic free hydroxyl groups and a hydrophobic alkyl chain.
Sugar surfactants are distinguished from other surfactants by their excellent detergent properties, non-toxicity, skin compatibility, environmental compatibility, and biodegradability. For these reasons, sugar surfactants are acquiring increasing significance.
Glucose is a monosaccharide having the following chemical structure: ##STR1##
Most of the approaches in synthesizing sugar surfactants have been directed to the preparation of fatty acid esters of mono- and disaccharides. Vlahov I. R., et al., Chem. 1997, 16, 1 and references cited therein. Attempts to synthesize cyclic sulfates of unprotected sugars with sulfuryl chloride and pyridine have been reported. Bragg P. D. et al, Can. J. Chem. 1959, 37, 1412. However, this reaction is not clean, and several side products are isolated.
There is therefore a need in the art for a means of synthesizing regiospecific sugar-based surfactants in an efficient manner.
The present inventors have now discovered that the regioselective chemical introduction of fatty acid or fatty amine groups into glucose leads to surface-active neutral glucose esters.
It is therefore a primary objective of the present invention to provide a method and means of synthesizing glucose-based surfactants in high yields.
It is a further objective of the present invention to provide a method and means of synthesizing glucose-based surfactants that is regiospecific.
It is still a further objective of the present invention to provide a method and means of synthesizing glucose-based surfactants wherein the surfactants have excellent surface active properties.
It is yet a further objective of the present invention to provide a method and means of synthesizing glucose-based surfactants that is efficient and economical.
These and other objectives will become clear from the following detailed description of the invention.